This invention relates to a novel polyimide-siloxane precursor, a process for producing the same, and a cross-linked polyimide-siloxane.
Polyimide resins have so far been widely used for protecting materials, insulating materials and adhesives in the field of electronic equipment or films, structural materials, etc., mainly in the aspect of heat resistance. The process of using the resins has relied, in most cases, on a process of applying a precursor prior to forming a cyclized polymer, as it is, onto an object, followed by baking to thereby complete imidization and also effect cross-linking, and various proposals have been made for improving the functions and effects after baking suitable to the above-mentioned various uses. Such prior art, however, cannot always be said to sufficiently satisfy current diversified, indivisualized and sophisticated needs.
For example, polyamic acids which have so far been used for a polyimide precursor for electronic materials have been applied in the form of their solution on a substrate, followed by baking to effect imidization and curing, but at the time of their use, various problems have been raised such that baking requires as high a temperature as 300.degree.-400.degree. C. which often exceeds the heat-resistant temperature of the substrates; and adhesion of the coating solution onto silicon wafer, glass, etc. is insufficient; etc.
As to such adhesion among these problems, a number of copolymers of polyamic acid with silicon compounds have been proposed for improving the adhesion. For example, Japanese patent application laid-open Nos. Sho 57-143328/1982, Sho 58-7473/1983 and Sho 58-13631/1983 propose a technique that a polyimide precursor obtained by replacing a portion of a diamine component as raw material by a polysiloxane terminated with diamines at both the ends thereof is used to prepare a polyimide-siloxane copolymer. In this case, however, a problem has been raised that in place of improving the adhesion to a certain extent, the polymerization degree decreases with the increase of the siloxane content in the resulting copolymer to lower the coating-formability.
Further, Japanese patent publication Nos. Sho 58-32162/1983 and Sho 58-32163/1983 disclose a process wherein a suitable carboxylic acid derivative such as a tetracarboxylic acid dianhydride is reacted with a diamine, to form a polyamidecarboxylic acid having a terminal group such as acid anhydride. This polyamidecarboxylic acid is reacted with an aminosilicon compound at -20.degree. C. to +50.degree. C., to obtain a silicon-containing polyamidecarboxylic acid prepolymer (a precursor), which is not imidized or imidized (chemically cyclized) under mild conditions (low temperature, preferably 50.degree. C. or lower, particularly -20.degree. C. to +25.degree. C.) in the presence of a dehydrating agent to form an organic silicon-modified polyimide precursor. The former unimidized precursor or the latter polyimide precursor is heated in the form of a solution in the presence or absence of a silane diol or a siloxane diol to effect completion of imidization and also cross-linking, to thereby obtain a polyimide-siloxane precursor. However, this polyimide-siloxane precursor has raised various problems that in the case where it is not cyclized, it requires baking at a high temperature of about 200.degree. C. or higher, up to 350.degree. C. for imidizing it after coating as in the case of conventional polyimide precursor composed mainly of polyamidecarboxylic acid; if the resulting cyclized substance has a high silicon content, the coating formability is inferior, while if it has a low silicon content, adhesion onto silicon wafer, glass, etc. is inferior; and in the case where a pre-cyclized (preimidized) polyimide-siloxane precursor is prepared, cyclization by low temperature treatment in the presence of a dehydrating agent is carried out, but this requires a long time and hence is not practical, while if cyclization is promoted by heating, the whole solution gels to lose fluidity.
Further, the present inventors have previously proposed a soluble polyimide-siloxane precursor using a tetracarboxylic acid dianhydride, a diamine, an amino-silicon compound and a silylating agent as raw materials which are in part the same as those used in the present invention (Japanese patent application No. Sho 59-230428/1984). This precursor has superior, practical characteristics, but on the other hand, it has a drawback of being somewhat inferior in storage stability.
In view of the above various problems of the prior art, it has been desired to develop a precursor which is soluble in a suitable solvent; has a suitable viscosity in the form of a solution to afford good operability; can be baked and cured at a relatively low temperature and for a relatively short time; has a good coating-formability; has a superior storage stability; and has superior adhesion onto silicon wafer, glass, etc., so that the resulting solution may be suitable for surface-protection of semiconductors, insulating film between multilayer interconnections, etc.
The object of the present invention is to provide such a soluble polyimide-siloxane precursor which overcomes the above problems (in a first aspect), a process for producing the same (in a second aspect), and a cross-linked polyimide-siloxane (in a third aspect).